Two growth factor signalling pathways in fibroblasts distinguished by pertussis toxin

Abstract

The primary action of a family of mitogens including bombesin1,2, bradykinin3, vasopressin3,4 and α-thrombin5–7 is to activate the hydrolysis of polyphosphoinositides8. Hydrolysis of phosphatidyl-inositol 4,5-bisphosphate (PtdIns(4,5)P2) by phospholipase C is mediated through coupling of surface receptors to a GTP-binding protein9,10 (Gp protein) which, in some cells6,11,12, is inactivated by the toxin of Bordetella pertussis13. It is not known whether this signalling pathway is involved in initiating DNA replication, whereas it has been firmly established that reinitiation of DNA synthesis can be triggered without activation of PtdIns(4,5)P2 hydrolysis by, for example, EGF (epidermal growth factor) 14,15, FGF (fibroblast growth factor)16 and insulin/IGF-I (insulin-like growth factor-I)14, members of a class of mitogens known to activate receptor tyrosine kinases17,18. Taking advantage of the fact that Chinese hamster lung fibroblasts respond to either class of mitogens and that their Gp protein appears to be sensitive to pertussis toxin6, we have now analysed the toxin's effect on reinitiation of DNA synthesis and find that it inhibits up to 95% of thrombin-induced mitogenicity without affecting EGF- or FGF-induced DNA synthesis and proliferation. These findings strongly suggest that activation of PtdIns(4,5)P2-phospholipase C has a determinant function in growth control, and confirm the existence of alternative growth factor-signalling pathways independent of polyphosphoinositide breakdown.